1. Field of the Invention
The invention relates to a method of image processing which may be used in an image quality inspection unit such as an LCD (liquid crystal display) tester or CCD tester which utilizes an image to inspect an object being examined or in an image recognition apparatus in which an object being examined is recognized through an image thereof.
2. Description of the Related Art
A number of image processing techniques are available in the art in which an image is utilized in rendering an inspection, recognition, decision or diagnosis of or about an object being examined. Most of these techniques utilize digital images which are subject to digital image processing in a computer.
A digital image which is processed in an image quality inspection process as employed in an LCD tester or CCD tester comprises an assembly of picture elements in the image, each having a plurality of gradation levels such as 256 or 1024 gradations, for example, arrayed such that 640 picture elements are contained in the direction of X-axis while 480 picture elements are contained in the direction of Y-axis. If the image represents a uniform gray image information, for example, all the picture elements in the digital image have an equal pixel value or density value for an ideal image. However, in practice, noises are present on signal levels and pixel values contain a degree of variation from pixel to pixel, and there can be no instance that all the pixels have an equal value. Nevertheless, if the variation among the pixels remains to be on the order of .+-.20 gradation levels for an arrangement utilizing 1024 gradation levels, the image on the screen would appear clearly uniform to the human eye.
When CCD or LCD elements which form an image processor have defects which cause the variation in the pixel value to exceed the order of .+-.20 gradation levels, or more specifically, if a pixel or pixels in question exhibit a pixel value of 500 when surrounding pixels generally have pixel values around 400, it is immediately notable upon sight that there exists some abnormality.
Such a pixel or pixels are referred to as "defects". When there is only one pixel isolatedly which exhibits a difference in the pixel value greater than a certain magnitude as compared with surrounding pixels, it is referred to as a point defect, and when such point defects are aligned on a line, they are referred to as line defect.
In addition, if a pixel, as considered as a single point, exhibits a pixel value which is not significantly different from the surrounding and cannot be recognized as a point defect, there may be an assemlage of such pixels agglomerated together to exhibit pixel values which are slightly higher or lower locally than the surrounding. It is also possible that a variation in the pixel value is greater in one area than in the others. In such an instance, we recognize an "unevenness" in such assemblage or area. A defect presented in this manner by an assemblage of pixels is referred to as area defect.
In LCD panel, it is necessary to provide an orientation for the liquid crystal molecules, and at this end, an orientation film is formed in contact with a liquid crystal layer on the surface of a glass substrate. The manufacturing process of LCD panel includes a rubbing step in which the orientation film is rubbed by a cloth in a given direction. Molecules of the liquid crystal which is injected between a pair of glass substrates have their elongate axes aligned parallel to each other in the direction in which the orientation film is rubbed. If the array of the liquid crystal molecules is rendered uneven, there results an unevenness in the light transmission, and therefore it will be seen that the rubbing operation represents a very important step for LCD panel. However, the manner of rubbing may be streaked and uneven for some reason. When the resulting LCD panel displays a generally uniform white image, a streaked unevenness in the brightness occurs in the displayed image, which unevenness is referred to as "rubbing line".
The purpose of an image quality inspection unit is to defect such defects to render a decision as to whether the image is acceptable or faulty. The inspection unit utilizes an image processing to deliver outputs representing the location or area of such defects and other information.
In the prior art, a thresholding operation is used to detect a point or a line defect in an image which is subject to the image quality inspection. Because of the nature of the point or the line defect that it is fundamentally detected as a single point defect, a defective pixel exhibits a pixel value which is sufficiently greater or less than a variation in the pixel value which occurs in the surrounding. Accordingly, the thresholding operation simply establishes a threshold, which may be exceeded by a brighter defective pixel to locate a defect or which may not be reached by a darker defective pixel to locate another defect. In this manner, the number of such defects or the pixel values of individual defects are delivered.
By contrast, a more complicated processing is required to detect an area defect. This is because pixel values in an area which should result in an area defect are buried in a variation in the pixel value of an area which does not produce an area defect to prevent a determination on the basis of information relating to individual pixels whether a particular pixel value represents a defect or a noise. In the current practice, the area defect is detected by a processing procedure as mentioned below, for example.
Initially, a median filtering operation is applied to an original image A of an object being examined to provide a filtered image B from which noise components occuring as points are removed. A difference image C between the filtered image B and the original image A is formed, thus providing an image for individual points which comprises only noise components. A binarization of the difference image c provides a binary image D. During the binarization, a pixel having a value equal to or above a threshold and which may be determined to be a defect is designated as an active pixel having a pixel value of 1, while pixels below the threshold are designated as having a pixel value of 0. It will be seen that active pixels in the binary image D which have the pixel value of 1 may represent a noise, a point defect or a component of a lineal or area defect. In the binary image D, a difference noted between an area which represents an area defect and another area which is not resides in the fact that pixels having the pixel value of 1 are agglomerated in an area representing an area defect while such pixels are sparsely located in an area which does not represent an area defect. Accordingly, an image processing operation is applied so as to remove an isolated point, namely, a pixel in question which has the pixel value of 1 and which is surrounded by eight adjacent pixels which have the designated pixel value of 0.
A labelling operation is then applied to the image from which isolated points are removed, and concatenated pixels are grouped together. Specifically, pixels having the designated pixel value of 1 are searched for among the binary image to be labelled, and any concentrated pixel having the pixel value of 1 is designated by the same label. In this manner, pixels having the pixel value of 1 which are grouped together agglomerate in an area corresponding to an area defect, thus increasing the area of each label or the number of pixels which belong to the same group. The area is calculated for each label, and only those labels are left which have an area exceeding a given value. Since noise components which do not represent an area defect are eliminated, this processing operation allows an area defect to be detected.
The above technique of defecting an area defect is already established. However, the detection with the described image quality inspection unit is not easy to implement inasmuch as the rubbing line is a brightness unevenness of an extremely low contrast, which is experimentally determined to be on the order of 2 to 3%, and which occurs attributable to a slight difference in the force with which the orientation film is rubbed. Since the contrast of the rubbing line is comparable to noises, a lowering of the threshold permits noises to be detected, thus preventing the selective detection of only the rubbing line from being achieved.
When a rubbing line occurs, the streak frequently occurs across an extensive area over the LCD image. As compared with the extensive area over which the unevenness occurs, the image quality inspection unit described above is limited to a small area in its capability to detect an area defect. For a defect having an area of unevenness which is as large as one-third the screen size, for example, information relating to an area defect which extends over such an extensive area will be lost at the time the difference image C is formed according to the prior art. At present, there is no available technique of detecting a defect which presents an unevenness over such an extensive area of the screen.
It is an object of the invention to provide a method of image processing which detects an area defect over an extensive area of the screen by quantification.